Our recent studies show that nitric oxide synthase (NOS), the enzyme which catalyzes the conversion of L-arginine to citrulline with the liberation of nitric oxide (NO), is concentrated in sympathetic preganglionic neurons (SPNs). Since soluble guanylate cyclase (GC) is the receptor for NO, cells containing GC are thought to be the target cells of NO. In spite of the accepted concept that No is an inter- cellular signal messenger, our initial studies have localized GC- immunoreactivity to the spinal neurons. Therefore, we now propose to test the hypothesis that NO acts as an intracellular messenger in the SPNs. First, immunohistochemical methods will be used to determine whether or not GC and cyclic GMP are colocalized with NOS in the same SPNs. Positive results would confirm the presence of necessary enzymes for NO acting as an intracellular signal molecule. Second, pharmacological experiments will clarify whether or not excitatory (EPSPs) and inhibitory (IPSPs) synaptic potentials evoked in SPNs are modulated by agents that alter the NO formation in a predictable manner. Intracellular or whole-cell patch recordings will be made from SPNs in spinal cord slices harvested from 12-20 day old rats. Experiments are specifically designed to address the following questions. First, are N- methyl-D-aspartate (NMDA) and non-NMDA (AMPA/KA) excitatory amino acid receptor-mediated EPSPs differentially affected by agents that stimulate (eg. L-arginine and sodium nitroprusside) or inhibit (eg. NG-Methyl-L- arginine and hemoglobin) the formation of NO? Second, do agents that enhance or decrease NO formation modulate IPSPs mediated by glycine/GABA? Third, does cyclic GMP mimic the action of NO? Fourth, does NO exert its action on guanylate cyclase present in SPNs and/or in nerve endings presynaptic to SPNs? There is increasing evidence that NO may be involved in central regulation of sympathetic outflow. Hence, a comprehensive study of the action of NO on the electrical activity and synaptic transmission of SPNs should help to define the role of NO in maintaining optimal activity of SPNs. Conversely, excessive production or deficiency of NO in SPNs may lead to cardiovascular disorders such as essential hypertension and cerebrovascular occlusion.